1. Field of the Invention
The present invention relates to an optical (information) recording medium.
2. Discussion of the Background
Unexamined published Japanese patent applications Nos. (hereinafter referred to as JOP) 2003-48375, 2005-161831 and 2005-108396 describe teachings about write-once optical recording media in which information can be written and read for a wavelength shorter than blue laser wavelength. In these teachings, especially the utility of a recording layer containing a metal oxide or a half metal oxide, especially bismuth oxide, as a main component, is described. In addition, JOP 2005-064328 describes a write-once optical recording medium which has a recording layer having a bismuth oxide and containing at least one element X selected from B, P, Ga, As, Se, Tc, Pd, Ag, Sb, Te, W, Re, Os, Ir, Pt, Au, Hg, Tl, Po, At and Cd.
On the other hand, as the optical disc system in the blue laser generation, Blu-ray disc format in which an optical system having blue laser of 405 nm and an aperture NA of 0.85 is adopted is proposed and its commercialization has already started. In this format, different from the typical designing, irradiation of writing and reading light is designed to be not from the substrate side but from the side of a cover layer having a thickness of about 0.1 mm to secure the tilt margin in the optical system having a high NA.
However, in the case of a cover layer having a layer thickness of about 0.1 mm, when a disc is molded by the typical injection molding method for polycarbonate, a new problem arises such that sufficient mechanical strength and thickness distribution and uniformity of the optical property in the surface are not secured. Japanese patent No. 3241560 describes a structure in which a cover layer is provided on the opposite side to the typical case relative to a substrate. Namely, a reflective layer, a first dielectric layer, a phase change recording layer, a second dielectric layer and a cover layer are provided on a substrate in this order. Thereby, a phase change recording medium having a thin cover layer is manufactured. In this manufacturing method, a pre-grooved substrate is injection molded using a stamper and then a reflecting layer, a first dielectric layer, a phase change recording layer and a second dielectric layer are formed on the pre-grooved surface. Thereafter, on the surface of the second dielectric layer, a cover layer is formed by spin coating an ultraviolet curing resin or attaching a film sheet. As a method of obtaining a cover layer having a highly uniform layer thickness, JOP H11-213459 describes a method of applying an ultraviolet curing resin by a spin coating method in which the center hole is masked and JOP 2006-48496 describes a spin coat method in which the temperature distribution is imparted in a concentric manner on a substrate and the viscosity of an ultraviolet curing resin is made different along the radius direction.
In the thus made optical recording media, the writing and reading laser is incident from the reverse side of the substrate so that the thickness of the substrate can be sufficiently made to be thick. In addition, since substrates are not required to have particular optical characteristics such as transmittance or double refraction, such substrates can be molded with a focus on transferability of groove, mechanical characteristics of a medium, etc. When this type of optical recording medium is used, it is possible to use a head for an optical recording medium having a high NA while sufficiently securing the mechanical strength of a substrate.
However, when a cover layer is formed using an ultraviolet curing resin, the reliability of an optical information recording medium (optical recording medium) tends to deteriorate. Specifically, in the high temperature (80° C.) and high humidity (85% RH) acceleration test (300 h), the reading jitter (hereinafter referred to as archival jitter) of the initially recorded portion and the writing jitter (hereinafter referred to as shelf jitter) of the portion in which information is written after the test tend to deteriorate.
To the contrary, JOPs 2001-49198, H10-120982, H10-8018 and H09-169956 describe a technology in which a reliable adhesive structure is obtained by using a photo polymerization initiator having a relatively large absorption constant in a long wavelength range longer than about 400 nm to improve internal hardness. However, there is no description in any of the JOPs about application to the cover layer of an optical recording medium in which light is incident via the cover layer as in the present invention. In addition, the technology described in these JOPs is about read only media and thus improvement on archival characteristics and/or shelf characteristics of recordable media is not described. Also, there is no indication or description about a preferred range of the inner hardness.